The present invention relates to a coil for use in the probe of a nuclear magnetic resonance spectrometer and, more particularly, to a coil structure that is very sensitive.
In an NMR spectrometer, a probe holds a sample placed in a uniform polarizing magnetic field. A coil is disposed close to the sample within the probe to apply an exciting RF magnetic field to the sample. The resultant resonance signal from the sample is picked up by the coil, and then it is fed to a receiver circuit. The output from the receiver circuit is furnished to a computer, which takes the Fourier transform of the signal to obtain an NMR spectrum. The sensitivity of the NMR spectrometer, which operates in this way, depends to a large measure on the degree of coupling between the coil and the sample. Therefore, great care is taken in designing the shape and structure of the coil. An NMR spectrometer employing a superconductive magnet has typically used a cylindrical saddle coil as shown in FIG. 1, where two spiral coil turns 1 and 2 are symmetrically arranged about the axis Z of a cylinder and on the curved surface of the cylinder of a radius R in which a sample tube holding a sample is diposed. The coil portion 1 consists of a pair of straight turns 1A extending parallel to the axis Z and a pair of arc-shaped portions 1B lying in the plane that is perpendicular to the axis Z. Similarly, the coil turn 2 consists of a pair of straight portions 2A and a pair of arc-shaped portions 2B. FIG. 2 is a cross-sectional view of the straight portions 1A and 2A taken on a plane perpendicular to the axis Z. It can be seen from this figure that the pairs of straight portions 1A and 2A are disposed on the periphery of the circle of the radius R in a symmetrical relation with respect to Y plane containing the axis Z. These coil portions set up an RF magnetic field in a plane which is parallel to X plane, contains the axis Z, and meets the Y plane at right angles.
The concept that underlies the prior art coil as mentioned above is that two spiral coil portions 1 and 2 of the same shape are disposed on the curved surface of a cylinder in a symmetrical relation with respect to the Y plane. Consequently, the number of turns of such a coil is 2, 4, 6, 8, or other even numbers. The prior art technique described thus far is disclosed in U.S. Pat. No. 4,398,149. In this respect, such an undesirable situation often takes place that two turns are not sufficient but four turns are excessive because of the relation of the resonance frequency to the inductance of the coil. If such a situation occurs, either a probe having two turns of coil or a probe having four turns of coil must be selected, giving unsatisfactory results.